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TYBSc Paper IV- USCH604 Analytical Chemistry COMPLEXOMETRIC TITRATIONS By Mr. P. B. Thakur

Name: Dr. Pramod B. Thakur Class: T.Y.B.Sc Subject: Analytical Chemistry Title of Topic: Complexometric Titrations. TYBSc Paper IV- USCH604 Analytical Chemistry COMPLEXOMETRIC TITRATIONS By Mr. P. B. Thakur. COMPLEXOMETRIC TITRATIONS

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TYBSc Paper IV- USCH604 Analytical Chemistry COMPLEXOMETRIC TITRATIONS By Mr. P. B. Thakur

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  1. Name: Dr. Pramod B. ThakurClass: T.Y.B.ScSubject: Analytical Chemistry Title of Topic: Complexometric Titrations

  2. TYBSc Paper IV- USCH604 Analytical Chemistry COMPLEXOMETRIC TITRATIONS By Mr. P. B. Thakur

  3. COMPLEXOMETRIC TITRATIONS • General introduction • EDTA titrations • Advantages and limitations of EDTA as the titrant • Absolute and conditional formation constants of • Metal-EDTA complex • Construction of Titration curve • Types of EDTA Titrations • Methods of increasing the selectivity of EDTA as a titrant • Metallachromic indicators • Theory Metallachromic indicators • Applications of Metallachromic indicators

  4. What is complex???? Complex consists of an metal ion surrounded by molecules or anions. How Complex forms???? Complex is formed by the combination of metal ion with electron donating group or nucleophile.

  5. Complexometric Titrations: • A complexometric titration is technique of volumetric analysis in which a soluble, undissociated, stiochimetric complex is formed during the addition of titrant to the sample solution. • In Complexometric titration the formation of a colored complex is used to indicate the end point of a titration. • Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution. • An indicator capable of producing an definite color change is usually used to detect the end-point of the titration.

  6. Which complexation reaction can be used as a volumetric technique???? • The reaction which reaches equilibrium rapidly after each portion of titrant is added. • The reactions in which interfering situations do not arise. e.g. The formation of several different complexes of the metal ion with the titrant, during the titration process. • The reaction in which a indicator is capable of locating equivalence point with fair accuracy is available. • Complexation titrations are particularly useful for the determination of a large number of metal ions in solution. In practice, the use of EDTA as a titrant is well established in such complexometric reactions.

  7. EDTA Titrations: • What is EDTA??? • EDTA is Ethylene Diamine Tetra Acetic acid. • It has four carboxyl groups and two amine groups.

  8. Commonly EDTA is represented in the acid form as H4Y. EDTA = H4Y Due to low solubility of acid form of EDTA in water, its disodium dihydrate EDTA salt i.e. Na2H2Y.2H2O is used disodium dihydrate EDTA = Na2H2Y.2H2O

  9. Different Forms of EDTA EDTA = H4Y

  10. EDTA has four carboxyl groups and two amine groups. EDTA is polydenated ligand as it donate its six lone pairs of electrons for the formation of coordinate covalent bonds with metal cations to form Metal-EDTA complex. Metal-EDTA complex Metal-EDTA complex

  11. Role of pH in EDTA titrations • EDTA titrations are carried out in buffered solution of • the metal ions to be estimated. • The use of proper pH is important and is related to • the stability constant of a metal-EDTA complex. • E.g. Alkaline pH is required for the metals having • low stability constant. • Low Alkaline to mild acidic pH is required for • the metals having high stability constant. • The dissociation reactions of acid form EDTA, H4Y • are also pH dependant. • pH is also an important criteria for the proper • functioning of the indicator substance. Thus it is very important to maintain the pH during the EDTA titrations

  12. Advantages of EDTA as titrant: • ETDA formstable complex with various metal ions. • The complexation occurs in a single step and hence the titration of the metal produce a sharp change in the metal ion concentration at the equivalence point. • 3. The Metal-EDTA complexes are all water soluble and hence all studies can be performed in aqueous media. • 4. EDTA forms1:1 complex with all metal ions irrespective of all charge on the metal ions. The stoichiometry is hence same for all metal ions. The reaction can be represented as: • Mn+ + H2Y-2 + H2O MY(n-4) + 2H3O+

  13. Limitations of EDTA as titrant: • Formation of insoluble hydroxides: • Many ETDA titrations are carried out under alkaline pH which may lead to formation of insoluble hydroxieds or basic salts that may compete with the complexation process . • 2. Lack of selectivity: • Since EDTA forms stable complexes with most of the metal ions, it lacks selectivity if it is used to estimate a single metal cations from a solution of mixture of metal ions.

  14. Absolute formation constant EDTA forms 1:1 complex with all metal ions irrespective of all charge on the metal ions. The stoichiometry is hence same for all metal ions. If fully uprotonated form of EDTA (Y-4) is react with metal ion as follows: M+n+ Y-4 MY(n-4)+ Then absolute formation constant KMY is given as, [MY(n-4)+] KMY = ................................eq 1 [M+n] [Y-4] KMY is also referred as absolute stability constant. Higher the value of KMY, higher is the stability of complex formed.

  15. Group I Titrated in pH8-11 Group II Titrated in pH 4-8 Group III Titrated in pH 1-4

  16. Conditional formation constant In absolute formation constant only fully uprotonated form of EDTA (Y-4) is taken consideration. EDTA is tetra protonic acid. It is represented as H4Y. The four different stages of dissociation of this tetra protonic acid can be represented as follows: H4Y H+ + H3Y- [H+] [H3Y-] Ka1 = = 1.0 X 10-2 [H4Y] [H+] [H2Y-2] H3Y- H+ + H2Y-2 Ka2 = = 2.2 X 10-3 [H3Y-] [H+] [HY-3] H2Y-2 H+ + HY-3 Ka3 = = 6.9 X 10-7 [H2Y-2] [H+] [Y-4] HY-3 H+ + Y- 4 Ka4 = = 5.5 X 10-11 [HY-3]

  17. Each of the equilibrium is pH dependent and Y-4 will predominates only in solution of very high pH. At lower pH value, the forms of EDTA HY-3 , H2Y-2 ,H3Y-1 ,H4Y may also be present. Then the total concentration of uncomplexed EDTA (CY) is given by sum of the all forms of EDTA CY =[Y-4] + [HY-3] + [H2Y-2] + [H3Y-] + [H4Y] [H+] [H2Y-2] [H+] [H3Y-] [H4Y] = [H3Y-] = Ka2 Ka1 [H+] [HY-3] [H+] [Y-4] [H2Y-2] = [HY-3] = Ka3 Ka4 Substituting the concentrations of the various species in terms of Ka1, Ka2, Ka3, Ka4 in above equation and solving the equation for the fraction Y-4 form, we get Ka1Ka2Ka3Ka4 [Y-4] = CY [H+]4 + [H+]3 Ka1 + [H+]2 Ka1 Ka2 + [H+] Ka1 Ka2Ka3 + Ka1 Ka2Ka3Ka4

  18. We may now define α4 as the fraction of EDTA in Y-4 form [Y-4] or α4 . CY = [Y-4] i.e. α4 = ...........eq 2 CY Substituting equation 1 in equation 2 [MY(n-4)+] KMY = [M+n] . α4 . CY [MY(n-4)+] = K’MY KMY . α4= [M+n] . CY K’MYis known as conditional formational constant, or conditional stability constant or effective stability constant

  19. Conditional formation constant (K’MY) is product of absolute • formation constant (KMY) and α4 . • K’MY= KMY . α4 • The Conditional formation constant (K’MY) is easily calculated • by knowing the absolute formation constant (KMY) and α4 • value at the pH of solution. • The Conditional formation constant (K’MY) is greater practical • use because it represents the actual tendency of EDTA to • form the complex with metal at the pH value of the solution. • The values of Conditional formation constant (K’MY) varies with • pH because α4 vary with pH.

  20. Values of α4 for EDTA at different pH As pH increases, α4 values also increases and Conditional formation constant (K’MY) is also increases

  21. Due to such important role of pH in complex • formation with EDTA , normally the solution of metal • ions are buffered so that the pH will remain constant. • In this way the Conditional formation constant (K’MY) • can be estimated and it is possible to calculate and • construct the titration curve from which it is possible • to judge the feasibility of the complexometric titration. • The pH is often adjusted to as low a value as is • consistent with feasibility. • The construction of the titration curve for metal-EDTA • titration can be given as below.

  22. Titration curve for metal-EDTA titrations In the Case of metal-EDTA titration, the EDTA is added to the buffered solution of metal and pM is calculated by using pM = -log10 [Mn+] during the titration. The obtained pM values is plotted on Y-axis against the volume of EDTA solution added on x axis. This curve give the point of inflexion at the equivalence point. The curve of pM agaist volume of EDTA added is known as Titration curve of metal-EDTA titrations. Example: Titration curve of Ca+2 with EDTA

  23. Example: Titration curve of Ca+2 with EDTA • Absolute formation constant (KMY) for CaY-2 is 5.0 X 1010 and • α4 at pH 10 is 0.35. • K’MY = KMY . α4 = 5.0 X 1010 X 0.35 = 1.8 X 1010 • Solution: • Take 10 cm3 of 0.01 M ca+2 solution buffered at pH 10 and • titrate with 0.01 M EDTA solution. • Calculate the pCa at different stages of titration such as, • At the start of the titration • After addition of 1.0 cm3 of titrant EDTA • At the equivalence point • After addition of 11.0 cm3 of titrant.

  24. At start of Titration: • At start of Titration when no titrant EDTA is added to the solution, only Ca+2 , will be present in the solution. • The concentration of Ca+2 can be given as, • [Ca+2] = 0.01 • pCa = -log10 [Ca+2] = -log10 [0.01] = 2.0 • pCa = 2.0 • 2.09

  25. 2. At the addition of 1.0 cm3 of titrant: When 1.0 cm3 of titrant EDTA is added to the solution, 1.0 cm3 Ca+2 will react with EDTA and it will form complex. The 9.0 cm3 of Ca+2 will remained unreacted in the solution and total volume of solution will become 11 cm3. The concentration of Ca+2 present in the solution can be given as, [Ca+2] = 0.01 X 9 pCa = -log10 [Ca+2] = -log10 [0.0082] = 2.09 pCa = 2.09 = 0.0082 M 11

  26. 3. At the equivalence point: (when 10 Cm3 of EDTA is added) When 10.0 cm3 of titrant EDTA is added to the solution, 10.0 cm3 Ca+2 will react with EDTA and it will form complex and total volume of solution will become 20 cm3. The concentration of Complex [CaY-2]present in the solution can be given as, [CaY-2]= 0.01 X 10 At equivalence point, concentration of Ca+2 will be equal to the concentration of EDTA . As EDTA is present in its all form. So the concentration of EDTA is given by CY [Ca+2] = CY = 0.005 M 20

  27. Conditional formation constant (K’MY) for CaY-2 is 108 X 1010, Now Conditional formation constant (K’MY) is [CaY-2] K’MY = = 1.8 X 1010 [Ca2+]. CY Since [Ca+2] = CY [CaY-2] K’MY = = 1.8 X 1010 [Ca2+]. [Ca+2] By substituting, K’MY = 1.8 X 1010 [CaY-2] = 0.005 In the above equation and we can calculate the concentration of [Ca2+]

  28. 0.005 1.8 X 1010 = [Ca2+]2 0.005 [Ca2+] = 1.8 X 1010 [Ca2+] = 5.2 X 10-7 pCa = -log10 [Ca+2] = -log10 [5.2 X 10-7] = 6.28 pCa = 6.28

  29. 4. After the addition of 11 Cm3 of EDTA: When 11.0 cm3 of titrant EDTA is added to the solution, 10.0 cm3 Ca+2 will react with EDTA and it will form complex and 1.0 cm3 of EDTA will remain excess in the solution. Total volume of solution will become 21 cm3. The concentration of Complex [CaY-2]present in the solution can be given as, [CaY-2]= 0.01 X 10 = 4.76 X 10-3 M 21

  30. The 1.0 cm3 of EDTA is excess, concentration of excess of EDTA present in the solution can be given as, [Excess of EDTA]= 0.01 X 1 The concentration of EDTA in the solution given by CY = 4.76 X 10-4 M 21 CY = 4.76 X 10-4 M

  31. Conditional formation constant (K’MY) for CaY-2 is 1.8 X 1010, Now Conditional formation constant (K’MY) is [CaY-2] K’MY = = 1.8 X 1010 [Ca2+]. CY By substituting, K’MY = 1.8 X 1010 [CaY-2] = 4.76 X 10-3 [CY] = 4.76 X 10-4 M In the above equation and we can calculate the concentration of [Ca2+]

  32. 4.76 X 10-3 1.8 X 1010 = [Ca2+]X4.76 X10-4 4.76 X 10-3 [Ca2+] = 1.8 X 1010 X4.76 X10-4 [Ca2+] = 5.55 X 10-10 pCa = -log10 [Ca+2] = -log10 [5.55 X 10-10] = 9.26 pCa = 9.26

  33. Table of pCa Value When the pCa is plotted against volume of titrant added, it gives the Familiar S shaped titration curve with sharp increse in the curve in the value of pCa in the vicinity of the equivalence point.

  34. Titration curve for Ca+2 - EDTA titrations pCa Volume of 0.01 M EDTA added

  35. Types of EDTA Titrations: EDTA Titrations Alkalimetric Titrations Indirect Titrations Direct Titrations Back Titrations Replacement/ Displacement/ Substitution Titrations

  36. Direct Titrations: • This is a direct determination of a metal ion by adding • standard EDTA titrant to the sample solution. • Metalion + EDTA [ Metal-EDTA ] complex • The solution containing the metal ion is buffered to the desired • pH and titrated directly with standard EDTA solution. • Some auxiliary complexing agent such as tartarate can be • added to prevent the prevent the precipitation of the hydroxide • of metal ion. • Cu+2, Zn+2, and Ni+2 can be determine by using direct • titration method. +

  37. Back Titrations: • This is method, an excess of standard EDTA is added to the • sample solution of metal ion. • The resulting solution will contain unreacted EDTA which is • then back titrated with standard metal ion solution in the • presence of indicator. • ZnCl2, ZnSO4, MgCl2, MgSO4 is used as standard metal ion • solution. • Al+3, Co+2, Pb+2, Mn+2, Hg+2, and Ni+2 can be determine by • using Back titration method

  38. Back Titration Process + + Sample Metal ion M1 Excess of standard EDTA Metal-EDTA Complex M1-EDTA Unreacted standard EDTA Standard Metal ion M2 Metal-EDTA Complex M2-EDTA

  39. Replacement, Displacement or Substitutions Titrations: • This is method, weak EDTA complex of another metal ion (M2) • is added to the solution of metal ion (M1) to be determined. • Mg-EDTA & Zn-EDTA are frequently used weak EDTA complex • The weaker metal EDTA complex is replaced with strong metal • EDTA complex. • The equivalent amount of metal M2 freed from the weaker • complex can be titrated with standard EDTA solution. • This method is useful for the determination of Ca+2 ion.

  40. Replacement, Displacement or Substitutions Titrations Process + + Replaced Free metal ion M2 Weak Metal-EDTA Complex M2-EDTA Strong Metal-EDTA Complex M1-EDTA Sample Metal ion M1 Standard EDTA solution Metal-EDTA Complex M2-EDTA

  41. Alkalimetric Titrations: • This method, use the principle of liberation of free H+ ions • during the complexation. • The reaction between metal ion and EDTA H2Y-2 produce H+. • M+n + H2Y-2 MY(n-4)+ + 2H+ • The free H+ ions is titrated with standard solution of alkali like • NaOH by using suitable acid-base indicator. • The H+ ions can also be determined by instrumental method.

  42. Alkalimetric Titrations Process + + 2H+ Sample Metal ion M+n standard EDTA H2Y-2 Metal-EDTA Complex MY(n-4)+ Liberated H+ ions NaOH Standard Alkali solution Acid-Base Titration product

  43. Indirect Titration: • This method is used to determine the ions such as Halides, phospates, and sulphates that do not form complex with EDTA . • In the determination of sulphate ion, SO4-2 ion solution is treated with excess of standard solution of Barium ion. • The formed precipitate of BaSO4 is filtered off and unreacted Barium ions present in filtrate is titrated with EDTA. • In this way, we are able to indirectly determine the amount of sulphate ion present in the sample solution.

  44. Indirect Titrations Process + Ba+2 + Anions like SO4-2 Excess of standard Barium solution BaSO4 Precipitate Unreacted Ba+ ions Standard EDTA solution Ba-EDTA Complex

  45. Methods of increasing the selectivity of EDTA as Titrations Methods Chemical Separation pH Control Masking Unmasking Kinetic Masking

  46. Chemical Separation • In this technique, the selectivity is increases by • separating the species from other components from the • sample solution. • The separated species is then dissolved in suitable • solvents and then titrated against EDTA using indicator. • Eg: Ca+2, Mg+2, Ni+2 and Cu+2 ions can be first separated as • Ca-oxalate, Ni-DMG, Mg-ammonium phosphate and • Cu-thiocyanide. • These precipitate are then dissolved in separate suitable • solvents and then titrate against EDTA using indicator.

  47. Chemical Separation Ca+2 Mg+2 Ni+2 Cu+2 Ammonium phosphate DMG thiocyanide Oxalate Ni-DMG precipitate Mg-ammonium Phosphate Precipitate Cu-thiocyanide precipitate Ca-oxalate precipitate

  48. Ca-oxalate Ni-DMG Mg-ammonium phosphate Cu-thiocyanide solvent Ca+2 solvent Ni+2 solvent Cu+2 solvent Mg+2 EDTA EDTA EDTA EDTA Ca-EDTA Ni-EDTA Mg-EDTA Cu-EDTA

  49. Control of acidity of pH of the solution • In this technique, the selectivity is increases by • controlling the pH of the solution. • The conditional formation constant for metal-EDTA • complex is depend on the hydrogen ion concentration. • So by adjusting the pH of the sample solution containing • several metal ions, it is possible to allow only a single • species to react with EDTA. • E.g.: Ca+2 can be determined in the presence of Mg+2 in • strongly alkaline solution (pH < 10). • Trivalent ions loke Bi+3, Fe+3 can be selectively • determined from the solution of bivalent metal ions in • strongly acidic solution (pH ~ 2.

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